Conventionally, polypropylene fiber has widely been used in the field of hygienic materials such as surface materials of disposable diapers, since it is lightweight, has excellent heat-retaining properties and softness, and is significantly advantageous in balance between performance and economy. In recent years, non-woven fabrics having physical properties of even higher toughness and softness are demanded for hygienic materials.
In order to obtain non-woven fabrics having higher toughness, melt-adhesion of polypropylene fibers must be improved. To this end, non-woven fabrics must be processed under conditions of high temperature so as to satisfactorily soften polypropylene fibers upon melt-adhesion of the non-woven fabrics by way of heat-rolling. However, high-temperature processing of non-woven fabrics thermally affects part of polypropylene fibers other than melt-adhesion sites to produce non-woven fabrics having sufficient tenacity but decreased softness. In contrast, low-temperature processing of non-woven fabrics results in poor melt-adhesion, and tenacity of the obtained non-woven fabric becomes low. There is also a problem that, when used under high-temperature conditions, non-woven fabrics or processed products thereof such as cylindrical filters have poor durability, decreased shape-retention, and decreased rigidity.
As described above, processing and use of conventional polypropylene fiber at high temperature are restricted due to the disadvantage of low heat resistance, and therefore improvement of performance of polypropylene fiber is strongly desired for development of further uses.
Moreover, conventional polypropylene fiber is difficult to be made micro-fine on account of the occurrence of fuming or thread breakage during spinning due to low-molecular-weight components contained in the polymer, and the level of ultra-fining is therefore limited.
Japanese Patent Application Laid-Open (kokai) No. 62-156310 discloses a polypropylene fiber endowed with excellent properties for heat-roll processing and formed of an ethylene-propylene random copolymer containing ethylene in a specific amount and having a softening point of 132.degree. C. or less. However, non-woven fabric made of this fiber has disadvantages of poor softness and a narrow temperature range that enables processing into non-woven fabrics having tenacity and softness suited for actual use.
Meanwhile, in recent years, J. A. Ewen et al. have found that syndiotactic polypropylene having a narrow molecular weight distribution and a high syndiotactic pentad fraction is obtained through polymerization of propylene by combined use of an asymmetric transition metal catalyst which differs from a conventional Ziegler catalyst and a metallocene catalyst comprising aluminoxane (J. Am. Chem. Soc., 110, 6255 (1989)). In connection with the metallocene catalyst, Japanese Patent Application Laid-Open (kokai) No. 3-82814 discloses a fiber formed of syndiotactic polypropylene having a syndiotactic pentad fraction of 0.7 or more. The polypropylene fiber disclosed therein is endowed with excellent softness, but heat resistance during melt-adhesion of fiber-bonding sites is particularly inferior to that of conventional polypropylene fibers.
Accordingly, no polypropylene fiber having satisfactory tenacity and softness has yet to be developed.